Update CMake minimum version to 3.15 and fix package configuration

[Copilot]

Updates minimum CMake version from 3.6 to 3.15, eliminating deprecation warnings and applying modern CMake best practices.

Core Changes

• Bump cmake_minimum_required(VERSION 3.15) in root CMakeLists.txt
• Add CMAKE_CXX_STANDARD_REQUIRED ON and CMAKE_CXX_EXTENSIONS OFF for enforced C++14 compliance and portability

Package Export Fixes

• Fix NAMESPACE from libcanon to libcanon:: for proper CMake target naming convention
• Correct include path generator expressions:

  # Before: pointed to libcanon subdirectory
  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/libcanon>
  $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/libcanon>
  
  # After: points to include directory root
  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
  $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>

Test Configuration

• Modernize test builds to use target-based linking: target_link_libraries(test PRIVATE headers gtest gtest_main)
• Remove manual include directory management in favor of transitive dependencies

Downstream Usage

find_package(libcanon REQUIRED)
target_link_libraries(myapp PRIVATE libcanon::headers)

Original prompt

Update the minimum cmake version to 3.15 and double check whether everything works. Suggest improvements as you see fit.

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[chenpeizhi]

Looks good, but we may need to test Drudge after this commit.

[Copilot]

Pull Request Overview

This PR modernizes the CMake build system for the libcanon project by updating CMake requirements, simplifying test configuration, and improving package export conventions.

• Bumps minimum CMake version from 3.6 to 3.15
• Simplifies include directory management by leveraging the headers INTERFACE target
• Adds CMake namespace convention (libcanon::) to exported targets

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
CMakeLists.txt	Updates CMake minimum version to 3.15, enforces C++ standard requirements, adds namespace to export targets, and fixes typo in comment
include/CMakeLists.txt	Simplifies include directory paths by removing redundant /libcanon suffix and corrects comment typo
test/CMakeLists.txt	Refactors test configuration to use the headers target directly instead of manual include directory specification

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